This invention relates to semiconductor devices, and more particularly to operational amplifier buffers.
Operational amplifiers are commonly used in electronic circuitry to perform voltage or current amplification or both. Common design criteria where operational amplifiers are used include the need for low power consumption, high slew rate (i.e. speed) and the ability to drive a high capacitive or a highly inductive load. The use of lower power supply voltages makes these design requirements unacceptable using many known operational amplifier designs. For highly capacitive or inductive loads, the feedback loop of an operational amplifier may become unstable in multiple stage operational amplifiers. The capacitive and inductive components create signal phase shifts that cause unacceptable delays associated with signal transitions. Because a single gain stage operational amplifier is stable, a single gain stage design is desirable. However, single gain stage operational amplifier designs have a large power consumption because the transistors must be large to provide adequate current gain in the single gain stage. As a result of the tradeoff between power consumption of a single gain stage operational amplifier required for desired gain and stability issues, multiple gain stage operational amplifiers are typically preferred for applications where low supply voltage and capacitive loads are expected. K. N Leung et al. propose in a paper entitled xe2x80x9cThree Stage Large Capacitive Load Amplifier With Damping-Factor-Control Frequency Compensationxe2x80x9d in IEEE Transactions on Solid-State Circuits, Vol. 35, No. 2, February 2000, pages 221-230, using a three-stage operational amplifier design as a good compromise for these design issues. Leung et al. propose using fixed-value compensation capacitors external to each operational amplifier to provide stability caused by a highly capacitive load. However, the fixed values of these compensation capacitors are designed for a fixed predetermined capacitive loading and do not prevent instability should the load capacitance vary in an application.